Steel Structures 5 (2005) 00-00 www.kssc.or.kr

Innovative Dissipative (INERD) Pin Connections for

Seismic Resistant Braced Frames

Ioannis Vayas* and Pavlos Thanopoulos

School of Civil Engineering, National Technical University of Athens, 15780 Zografou, Greece

Abstract

Innovative dissipative (INERD) connections were developed for seismic resistant braced frames. The dissipative zones insuch frames are the connections, while the braces are protected against buckling. Two types of INERD connections weredeveloped: pin connections and U-shape connections. This paper presents studies on the pin connections, where the braces areconnected to their adjacent members by means of eye-bars and a pin running through them. Experimental and theoreticalinvestigations show a high energy dissipation capacity of these connections that is due to the inelastic bending action of thepin. The susceptibility to brittle fracture or low-cycle fatigue is low as inelastic action takes place away from welds or stressconcentrations. Design rules for the connections are developed. The beneficial mechanical behaviour and other constructionaladvantages provide a promise for a wide application of the invented connections for buildings and engineering structures inseismic regions.

Key words: Connections, Dissipative, Steel, Braced Frames, Earthquakes

1. Introduction

Earthquake resistant steel frames are usually designed

so that they exhibit a dissipative structural behaviour. In

such a case, parts of the structure (dissipative zones)

exhibit inelastic deformations during strong seismic

motions. The main structural typologies (Mazzolani et

al., 2000), the correspondent performance characteristics

and the expected positions of the dissipative zones are

listed in Table 1. It may be seen that conventional frames

(columns 2 to 4) have certain disadvantages in respect to

stiffness or ductility. Additionally in such frames,

following problems arise after strong earthquakes due to

the position of the dissipative zones, where damage is

expected to concentrate: a) the need for strengthening or

replacement of damaged and buckled braces which have

a certain length and are difficult to handle, b) the need

for strengthening and repair of the links or the beams that

are part of the main system that supports gravity loading.

Such works require considerable skill and are associated

with high material and labour costs.

Damages in steel framed structures after recent strong

earthquakes indicate the need for improvement of

existing structural typologies and for introduction of

innovative systems. These systems shall have the

following properties: a) High stiffness in order to limit

drifts during moderate seismic motions, b) high ductility

in order to dissipate energy during strong motions and c)

possibility for easy inexpensive repair, if required. In the

present paper, a new system with such properties,

applicable mainly to concentric, but possibly also to

eccentric braced frames is presented (Table 1, column 5).

The system was developed and studied during a joint

European research project, involving 4 Universities

(Athens, Lisbon, Milan and Liege) and a steel production

Company (Arcelor/Arbed). Supplementary investigations

were performed during a national Greek research project,

involving the National Technical University of Athens

and 5 Software and Construction companies. A priority

European Patent Application has been filed on the

invented connections.

Braced frames with INERD-connections exhibit the

following benefits compared to conventional steel frames:

• Better compliance with the seismic design criteria

(Table 1, column 5).

• Protection of compression braces against buckling.

• Activation of all braces, either in compression or in

tension, even at large storey drifts.

• Limitation of inelastic action and damage in small

parts of the structure that may be easily replaced.

• Avoidance of brittle fracture and/or low-cycle fatigue.

• Possibility for easy inexpensive repair after very

strong earthquakes, if required.

• Reduction of overall structural costs for the same

performance level.*Corresponding authorTel: 0030 210 7721054E-mail vastahl@central.ntua.gr

2 Ioannis Vayas and Pavlos Thanopoulos

2. Description of the INERD Connections

According to the current European seismic rules

(Eurocode 8, 2004), “concentric braced frames shall be

designed so that yielding of the diagonals in tension will

take place before failure of the connections and before

yielding or buckling of the beams or columns” and that

“in frames with diagonal bracings, only the tension

diagonals shall be taken into account”. The former

condition leads to high connection costs for conventional

braced frames, since the connections shall be stronger

than the connected members and remain elastic during

the seismic excitation. The latter indicates that the

compression braces, almost half of the total, are

considered, due to buckling, as inactive, which evidently

leads to heavier brace sections and higher costs.

However, Eurocode 8 leaves the door open for the

development of innovative dissipative connections, as it

states that “The overstrength condition for connections

need not apply if the connections are designed to

contribute significantly to the energy dissipation capability

inherent to the chosen q-factor and if the effects of such

connections on the behaviour of the structure are

assessed”. The hereafter presented INERD connections

fall into the above category and are therefore weaker

than the connected members, exhibiting inelastic

deformations and dissipating energy during seismic

loading. Two types of INERD connections connecting

the braces to the adjacent members were developed: a)

pin connections and b) U-connections.

The pin connections consist of two external eye-bars

welded or bolted to the adjacent member (column for X-

braces, beam for V or eccentric braces), one or two

internal eye-bars welded or bolted to the brace and a pin

running through the eye-bars, as indicatively shown in

Fig. 1. Inelastic deformations and energy dissipation

concentrate in the pins. The pin cross section is not round

in order to avoid twist around its axis during cyclic

loading. Accordingly, two pin cross sections were

selected: a) either rectangular, where the pin is bent

around its small side (in order to avoid possible lateral

buckling), or b) rectangular with rounded edges, where

the pin is bent around its large side.

The U-connections consist of U-shaped thick plates

that connect the brace to the adjacent member, with one

leg parallel or perpendicular to the brace axis, as shown

in Fig. 2 where the brace load is applied horizontally.

Here again, energy dissipation takes place in the bent

plates. The advantage of these connections is that, by

appropriate sizing, inelastic deformations are limited

within exactly predetermined zones, the pins or the U-

plates, whereas the adjacent parts remain elastic.

Consequently, damage takes place away from welds or

notches and is restricted to the pins or the U-plates that

may be easily replaced, if largely deformed, after a

strong earthquake.

The study of the performance of the new system

included experimental and theoretical investigations, as

following:

• Full-scale tests on INERD connection details performed

in Lisbon (Calado and Ferreira, 2004)

• Full-scale tests on frames with INERD connections

performed in Milan (Castiglioni et al., 2004)

• Analysis of INERD pin connections performed in

Athens (Vayas et al., 2004)

• Analysis of X-braced frames with INERD pin

connections (Vayas et al., 2004)

Table 1. Structural typologies and main characteristics for Steel Frames

1 2 3 4 5

Moment resisting frames(MRF)

Concentric braced frames(CBF)

Eccentric braced frames(EBF)

CBF or EBF with INERD connections

Stiffness Low High Moderate High

Ductility High Low Moderate High

Dissipative zones at Beams Braces Link beams Connections

Figure 1. Rectangular pin connection with one or two internal eye-bars.

. |.

. |.

. |.

. |.

Innovative Dissipative (INERD) Pin Connections for Seismic Resistant Braced Frames 3

The present paper is focused on the analyses of the pin

connections. For details of the experimental investigations

reference is made to the relevant research reports

indicated above.

3. FEM Analyses for Cyclic Loading

The behaviour of the pin INERD connections has been

studied by FEM analyses that provide useful information

for the monotonic and cyclic response of the connections

at large inelastic deformations. The analyses were made

using the general purpose programme ABAQUS, version

6.4. The contact between the eye-bars and the pin, was

modelled by applying special interaction properties

between the appropriate surfaces, as ABAQUS provides

a vast variety of contact properties (e.g. stiffness, friction

etc.) to select from. Making use of the double symmetry

properties allowed for modelling of one quarter of the

connection that included one half of an external and one

half of an internal eye-bar and a quarter of the pin.

Monotonic loads were applied in the analysis through

axial displacement control up to 50 mm. Cyclic loading

was applied in cycles, the amplitude of which increased

by 5 mm from that of the previous ones. The analyses

were made in a first step for the connections that were

tested experimentally by Calado and Ferreira in Lisbon,

in order to allow a validation of the relevant models.

Accordingly four configurations with two internal eye-

bars were analyzed, together with two extra ones with

one internal eye-bar that were investigated only

analytically (Table 2). It may be mentioned that the

cyclic tests were performed according to the ECCS

testing procedure with three equal full cycles at the

prescribed displacements, ECCS 1986, while in the

analysis one full cycle was applied.

Figure 3 shows the connection at large displacements,

together with the von Mises stresses, indicating:

a) the spread of plasticity in the pin around the internal

eye-bars

b) the plastic deformations at the inner side of the

external eye-bars

c) the hole ovalisation in the eye-bars and

d) the transverse deformations, especially of the

thinner eye-bars

Figure 4 shows the response of the connection type D

as determined by the tests and the FEM analysis. The

material stress vs. strain law was taken such that allowed

for the inclusion of Bauschinger effects which appeared

to be important. The friction coefficient between the pin

and the eye-bars was taken equal to 0.4. It may be seen

that the connection strength to positive loading (eye-bars

in compression) is higher than the relevant strength to

negative loading (eye-bars in tension) for reasons to be

explained later. Some pinching is observed due to

ovalisation of the holes of the eye-bars, otherwise stable

hysteretic loops are achieved. Similar satisfactory

agreement between experimental and FEM results was

observed for all types of tested connections. It may be

stated that the analyses and the tests indicated that the

monotonic curves represent skeleton curves of the cyclic

ones, except at low deformations where they are stiffer

than the latter.

Figure 5 shows FEM results of the same connection

without consideration of Bauschinger effects (bilinear

material law). It may be seen that this analysis does not

correctly express the connection response in that it

predicts initiation of slipping at almost constant forces.

As previously mentioned, the eye-bars tend to exhibit

transverse inelastic deformations during cyclic loading.

Figure 6 shows analysis results for these deformations

for the connection type D and a picture of the connection

after the test, where these deformations are visible. It

may be seen that: a) the external eye-bars deform outwards,

while the internal inwards, b) these deformations are

accumulating in the cycles, increasing thus both the

overall span (distance between external eye-bars) and the

internal span (distance between external and internal eye-

bars) of the pin, c) the transverse deformations are higher

for the, thinner, internal eye-bars than the, thicker,

external ones. Obviously if only one internal eye-bar is

used (Table 2, types E and F) the relevant transverse

deformations disappear due to symmetry.

The applied moment on the pin, and therefore the

connection strength, is linearly varying with the distance

Figure 2. U-connection with one leg parallel or perpendicular to the brace axis (load horizontally applied).

. |.

4 Ioannis Vayas and Pavlos Thanopoulos

between external and internal eye-bars. This is confirmed

by the FEM analyses (Fig. 7), that show the response of

the connection types B and D (Table 2), as well as the

connection type F with one internal eye-bar whose

thickness is equal to the thickness of the external ones (=

30 mm).

The dissipated energy of the above connections is

shown if Fig. 8. It may be seen that the connections are

dissipating large amounts of energy even al large

displacements. The dissipation capacity is higher for

stronger connections (smaller distance between eye-

bars). But connections with one internal eye-bar, type F,

possess also a high dissipation capacity, comparable to

those with two eye-bars. By varying the number and

distance of eye-bars, the connections may be designed

according to the strength and dissipation demand of the

structure under consideration.

Subsequently, the dissipation capacity of braced frames

with and without INERD connections was compared. For

this purpose conventional X-braced frames in which

energy dissipation takes place through inelastic action of

the tension diagonal were studied. The inelastic brace

response to cyclic loading was modelled by means of

experimentally derived curves (Black et al., 1980), as

shown in Fig. 9. The overall response of the X-braced

frame results obviously from the addition of the two

Table 2. Configurations used for testing and FE analysis

Innovative Dissipative (INERD) Pin Connections for Seismic Resistant Braced Frames 5

brace responses, one in compression one in tension. In

the frame with the INERD connections, all four

connections are participating in the energy dissipation.

Three frame configurations were studied: a) a frame

with dissipative INERD connections type B, b) a frame

with non-dissipative connections with moderate brace

slenderness and c) like b), but with larger brace

slenderness. For comparison, the braces in cases b) and

c) exhibited equal compression strength which was equal

or slightly less than the connection strength of case a).

Figure 10a shows the cumulative energy dissipation of

the three cases. It may be seen that the dissipation

capacity of the frame with INERD connections is much

higher than for the conventional frames where the

dissipative elements are the braces. The energy dissipation

for the more slender brace (λ = 1,08) is higher than the

correspondent one of the more compact brace (λ = 0,85).

This is due to the fact that both braces have equal

compression strength, so that the former has a larger

section. However, if the energy is normalized by the

tension capacity (Fig. 10b), as an expression of the

structural weight, it is higher for the compact brace and

lower for the slender one. It may be seen that the

dissipation capacity for the frame with the INERD

connection is even more pronounced in normalized terms

compared with the conventional braced frame.

Some allowance of holes in the eye-bars is required for

constructional reasons, in order for the pin to pass

through them. Figure 11 shows analyses results for

connection type B with 1 and 2 mm allowance. It may be

seen that a smaller allowance for holes results in a better

performance, especially at the initial loading cycles and

Figure 3. FE model for the pin connection in the deformed state at large displacements (1/4 of connection).

Figure 4. Experimental vs. FEM results of connection Type D.

6 Ioannis Vayas and Pavlos Thanopoulos

in the compression side. However, at larger cycles the

differences gradually disappear. Accordingly, a 2 mm

maximal allowance for holes should be permitted for

practical applications.

It may be added here that constant amplitude cyclic

tests until fracture were carried out in Lisbon by Calado

& Ferreira, 2004. Evidently, the number of cycles to

fracture was a function of the level of applied

deformations. The tests indicated a low susceptibility of

INERD pin connections to fatigue and fracture. This is

due to the fact that fracture takes place near the loading

points, away from welds, notches or other discontinuities.

4. Design Rules for the INERD Connections

In order to develop design rules for the INERD

connections, parametric FEM studies for monotonic

loading, which provide skeleton curves, were preformed.

Here the pin dimensions, the plate thicknesses, the

material properties for the pins and the eye-bars and

other properties were investigated. In this section results

for connections with two internal eye-bars will be

presented as the studies with one internal plate are still in

progress. Figure 12 shows the response of one such

group of connections in which the pin material has the

same yield strength as the plates. Each curve corresponds

to a different thickness of the external eye-bars, while the

thickness of the internal plates remains constant and

equal to 15 mm.

The connection response in the initial loading stages

corresponds to that of a beam subjected to four-point

bending. After the formation of two plastic hinges under

the loading points, the beam becomes theoretically a

mechanism. However, the external eye-bars provide a

“clamping” effect to the pin which is higher for thicker

external plates. The load can thus be further increased,

Figure 5. FEM results of connection in Fig. 4, without consideration of Bauschinger effects.

Figure 6. Lateral deformations of eye-bars.

Innovative Dissipative (INERD) Pin Connections for Seismic Resistant Braced Frames 7

up to the formation of two additional plastic hinges at the

supports. At that point the system becomes a plastic

mechanism and only strain hardening may contribute to

any further load increase. The above observations lead to

the conclusion that energy dissipation is primarily due to

the clamping effect of the external plates (which result in

an increase of strength up to 100% of the initial yield

resistance) and to a less extend to strain hardening. The

parametric studies, see also Fig. 12, indicate that the

connection strength above the yield point does not

increase linearly with the plate thickness due the

clamping effect. Particularly, as the stiffness of the

external eye-bars increases, the behaviour of the

connection approaches an envelope curve which

corresponds to eye-bars of infinite stiffness (i.e.

transversal bending of the eye-bars becomes negligible).

For a required level of connection strength the question

arises on the best selection of the pin dimensions and

material (smaller pin of higher material strength or larger

pin with lower material strength?). Figure 13 shows the

connection response for different pin materials, which

shows that the connection strength is primarily influenced

from the pin strength. The results of the parametric

studies indicate that it is better to select a smaller pin of a

higher strength. However the strength of the pin material

should not be higher than that of the plates. Under these

conditions, it is recommended to choose thicknesses of

the external plates ~0,75 to 1,0 times the smaller

Figure 7. Response of connections B, D (two internal eye-bars) and F (one internal eye-bar).

Figure 8. Dissipated energy of connections B, D and F.

8 Ioannis Vayas and Pavlos Thanopoulos

dimension of the pin section (around which the pin is

bent). The thickness of the internal eye-bars may be then

set equal to half of the external ones. Of course the plates

are dimensioned by application of capacity design

criteria for the plates, which require sufficient gross and

net section over-strength capacity in relation to the pin.

The tension strength of the connection is, due to

transverse bending of the plates, lower than the

compression strength. Conversely, the ratio of these

strengths provides an indication of the amount of this

bending, which, as stated before, influences negatively

the cyclic response as it is accumulating for cyclic

loading (Fig. 6). However, if the above recommendations

for the plate thicknesses are kept, this influence is low

and the strength ratio (tension to compression) is above

90%.

By means of engineering models and comparison with

the results of the parametric studies, simple formulae

appropriate for practical use were derived that allow for

the correct prediction of the connection response. Table 3

provides the relevant design formulae for the connection

with two internal plates (Fig. 1b) which were validated

against the experimental and theoretical results. In

addition, the von Mises stresses at Points I and II are

shown. Formulae for one internal plate (Fig. 1a) will be

proposed as soon as the relevant experimental investigations

will be finalized. The proposed formulae, which exhibit

an accuracy range of ±5% compared with the parametric

studies, are based on following mechanical models:

• Up to the yield load the pin behaves as a beam

subjected to four-point bending. The yield load and

yield deformation are derived on the condition of the

formation of plastic hinges below the load application

points. The reduction factor 1,1 on the distance “a”

between plates for the yield load represents excessive

yielding of the pin within the clear distance between

plates (Table 3, stresses at point I). The additional

factor 1,5 on the yield displacements represents

allowance from holes and the developments of some

inelastic deformations.

• The connection strength is derived from the ultimate

condition that corresponds to the formation of four

Figure 9. Cyclic response of a single brace.

Figure 10. Cumulative dissipated energy for X-braced frames with and without INERD connections.

Innovative Dissipative (INERD) Pin Connections for Seismic Resistant Braced Frames 9

plastic hinges in the pin (Table 3, stresses at point II).

• For tension loads, a 10% reduction is proposed due to

the increased span caused by the transverse

deformations of the plates.

• An over-strength factor of 1,25 is recommended for

the capacity design checks of the plates. This

excessive strength results in from strain hardening

effects at large deformations.

• The deformation capacity of the connection is very

high. However a limitation of this capacity to the

distance between external and internal eye-bars is

recommended.

The design rules for the connection may be summarized

as following:

• Connection strength according to Table 3

• Strength of pin material equal or less than the

material of the plates

• Thickness of external plates 0,75-1,0 times the

smaller pin dimension

• Thickness of the internal plates 0,5 times the

thickness of the external ones

• Connection deformation capacity equal to the clear

distance between external and internal plates

• Allowance for holes in the eye-bars 2 mm.

• Application of capacity design criteria on the

connection strength for dimensioning of internal and

external plates (over-strength factor 1,25)

As seen in Fig. 1, the pin length is fixed by the height

of the column section to which the external plates are

attached, or alternatively, by the width of the flanges, if

Figure 11. Response of connections B with different allowance for holes.

Figure 12. Response of INERD connections for various thicknesses of the external plates (tinternal plate = 15mm).

. |.

. |.

. |.

. |.

. |.

. |.

10 Ioannis Vayas and Pavlos Thanopoulos

Figure 13. Response of INERD connections for various pin materials (tinternal plate = 15 mm).

Table 3. Design formulae for the connection with 2 internal plates

Eye-bars in Force Displacement

Point I“yielding y”

Compression

Point II (Strength)“ultimate u”

Compression (Deformation capacity = a)

Points I and II Tension 90% of the above values for Py and Pu

Over-strength for capacity design checks

25% beyond Pu

Mp =Wpl · fy α = a/l

l = pin length (axial distance between external eye-bars)h = pin heighta = clear distance between internal and external eye-barsfy = yield stress of pinWpl = plastic modulus of pin cross section

Py

2 Mp

⋅

a 1 1,⁄( )-----------------= δ

y1 5,

Mp

E I⋅-------- l

2 α

6--- 3 4α–( )⋅ ⋅ ⋅ ⋅=

Pu

4 Mp

⋅

a------------=

| a

| a

| a

Innovative Dissipative (INERD) Pin Connections for Seismic Resistant Braced Frames 11

the column is turned by 900 compared with Fig. 1.

5. Conclusions

Two types of innovative dissipative (INERD) connections,

pin- and U-shaped, were developed for seismic resistant

braced frames. The connections and more specifically the

pins or the U-plates are the dissipative elements of the

frames. The advantages of the pin INERD connections

for which a priority European Patent Application has

been filed may be summarized as following:

• High stiffness for low loads, high ductility for higher

loads.

• Protection of braces against buckling.

• All braces, either in compression or in tension,

remain active even at large storey drifts.

• Limitation of inelastic action and damage in the pins

that may be easily replaced if required.

• Avoidance of brittle fracture and/or low-cycle fatigue.

• Reduction of overall structural costs for the same

performance level.

References

Black, R.G., Wenger, W.A. and Popov, E.P.. Inelastic Buck-

ling of Steel Struts Under Cyclic Load Reversal. Report

No. UCB/EERC-80/40. Berkeley: Earth. Eng. Research

Center. Univ. of California, 1980

Calado L. and Ferreira J. INERD Connections, Technical

Report, IST Lisbon, 2004

Castiglioni C, Crespi A., Brescianini J. and Lazzarotto L.

INERD Connections, Technical Report, Politecnico Mil-

ano, 2004

EN 1998, Eurocode 8, Design of structures for earthquake

resistance. CEN, European Committee for Standardisa-

tion, 2004

European Convention for Constructional Steelwork (ECCS).

“Recommended testing procedure for assessing the

behaviour of structural steel elements under cyclic loads”

ECCS Publ. No 45, Rotterdam, The Netherlands, 1986

F. Mazzolani, V. Gioncu (eds), Seismic Resistant Steel

Structures, Springer Verlag, 2000

Vayas I., Thanopoulos P. and Dasiou M. INERD Connec-

tions, Technical Report, NTU Athens, 2004